1. Field of the Invention
This invention relates to a display device within the finder of a camera, and more particularly to a display device suited for Albada finders.
2. Description of the Prior Art
The continuing trend to minimizing the bulk and size of cameras introduces an increasing demand to reduce the size of the camera finders. This is especially true in leaf shutter type cameras, or mid-price cameras, where an automatic focus detector is provided in the interior of the camera and the space needed for the detector further decreases the proportion of the space available for the finder in the camera. For this reason, examples of a mark finder that occupies a large space in mid-price auto-focus cameras has become very rare in recent years, and most of the cameras of this class employ an Albada type of finder.
FIG. 1 is a longitudinal section view of the optical system of a conventional Albada finder with an objective lens 1 of negative refractive power, a half-permeable concave lens 2 constituting a half-permeable reflection layer provided with a concave surface on the observation side, and an eyepiece 3. Reference numeral 4 indicates the position of an observer's pupil.
To reduce the size of this Albada finder, narrowing the separation between the half-permeable concave lens 2 and the eyepiece 3 seems logical. However, the reduction of the distance between the half-permeable concave lens 2 and the eyepiece 3 results in the following problem.
FIG. 2 illustrates an optical system where the half-permeable concave lens and the eyepiece are widely spaced from each other, and FIG. 3 another optical system where they are narrowly spaced. In FIGS. 2 and 3, the same parts as those shown in FIG. 1 are labelled with the identical reference numerals and their description is omitted here. Reference numeral 5 denotes a part of a reflective viewfield limit frame for displaying a photographic area, and "a", "b" and "c" represent, respectively, an upper marginal ray, a principal ray, and a lower marginal ray of a beam incident on the eye's pupil.
In comparing FIGS. 2 and 3, when the space between the half-permeable concave lens and the eyepiece is wide, as shown in FIG. 2, the reflective viewfield limiting frame 5 lies between the principal ray "b" and the lower marginal ray "c" at a point near the lower marginal ray "c". On the other hand, when that space is narrow, as shown in FIG. 3, frame 5 lies between the principal ray "b" and the lower marginal ray "c" at a point near the principal ray "b".
One of the inventors of the present invention has developed a display device for use within the finder. This is disclosed in U.S. patent application Ser. No. 279,221, filed June 30, 1981, now abandoned, which is succeeded by file wrapper continuation U.S. patent application Ser. No. 471,720, now U.S. Pat. No. 4,530,583. Its outline is shown in FIG. 4, where the objective lens is excluded from the illustration. The device includes a prism for illumination, a light source 7 and a display plate 8 to be displayed within the finder. The remaining reference numerals denote parts similar to those labelled the same reference numerals in FIGS. 2 and 3.
The inside-finder display device of FIG. 4 operates such that light from the light source 7 passes through the prism 6 to illuminate the display plate 8. Light from the illuminated display plate 8, after having been reflected from that surface of the half-permeable concave lens 2 on the observation side, passes through the eyepiece 3 to reach the pupil 4. Let us now consider the difference produced when this display device is applied to the two finders of FIGS. 2 and 3.
In observing through the finder, it is preferable that the display within the finder, for example, the object distance display, appear near the viewfield limiting frame 5, from the standpoint of convenience in relation to the object image to be observed. For this purpose, the display plate 8 must not be too far from the reflective viewfield limiting frame 5. In FIG. 4, it is not desirable that the display plate 8 be significantly spaced downward and away from the reflective viewfield limiting frame 5.
In the finder of FIG. 2, even when the display plate 8 and the prism 6 for illumination are arranged adjacent the reflective viewfield limiting frame 5, (the prism 6 for illumination is located slightly below the reflective viewfield limiting frame 5 as shown in FIG. 4), the rays "a", "b", and "c" are not all blocked by the prism 6 and are capable of reaching the pupil 4. In the finder of FIG. 3 where the half-permeable concave lens 2 and the eyepiece 3 are narrowly spaced, the reflective viewfield limiting frame 5 lies near the principal ray "b" and the lower marginal ray "c" is blocked by the prism 6 arranged adjacent the reflective viewfield limiting frame 5, as shown in FIG. 5. Additionally, the light beam from the display plate 8 illuminated by the light source 7 is partially blocked by the illuminating prism 6. For this reason, the arrangement of FIG. 5 makes the inside-finder very unclear.